The present invention is directed to a method for producing a single crystal according to the Czochralski method (hereinafter referred to as the "CZ method") or the liquid capsule Czochralski method (hereinafter referred to as the "LEC method") and more particularly to a method for controlling a plurality of vertically arranged heaters during the crystal pulling operation.
According to the well known CZ method, as illustrated in FIG. 1, a melt 3 of molten crystal material is placed in a crucible 1 which is maintained at the desired temperature by heating means 22 which are located in a conventional furnace. The surface of the melt is covered with a molten solution of B.sub.2 O.sub.3 (not shown) if the LEC method is being followed. A seed crystal 4 is inserted into the surface of the melt 3 and is thereafter pulled upwardly from the melt to produce a single crystal 5. During the crystal pulling operation it is necessary to maintain an absolute value of the upward temperature gradient small in the furnace according to a predetermined pattern to decrease an average dislocation density. Further, in this operation, the control of the heating means 22 within the furnace is very important to maintain the diameter of the crystal 5 constant.
As shown in FIG. 1, the heating means 22 is comprised of an upper heater 23 and a lower heater 24 disposed in vertical orientation relative to each other. Previously, in order to control the temperature gradient, a temperature control method was utilized for controlling the actual temperature of each heater or a power control method was utilized for controlling the power supplied to each heater. However, these methods suffered from the following problems:
(1) Since the temperature gradient is an intricate function heater temperature it is extremely difficult to detect variations of the temperature gradient in the crystal. PA1 (2) Since a great deal of heat flows between the upper and lower heaters 23 and 24 by convection, the temperature of the heaters are too strongly related to each other to perform the desired control function. PA1 (3) Since the temperature of the heaters are correlated to each other as described above, if the power for the lower heater is decreased with the temperature of the upper heater 23 maintained unchanged, then the power for the upper heater 23 is increased in order to maintain the temperature of the upper heater 23 unchanged. Therefore, the crystal is heated so that the diameter of the crystal is decreased. In other words, if the temperature of the upper or lower heater is decreased, the diameter of the crystal is decreased. On the contrary, if the temperature of the upper or lower heater is increased, the diameter of the crystal may be increased. That is, in general, if the temperature of the crystal is decreased, the diameter thereof tends to be increased. However, it is difficult to control the diameter of the crystal by using the control method as described above. PA1 (4) In the power control method it is rather difficult to satisfactorily deal with variations in pressure or variations in the furnace wall temperature. In the heater temperature control method the variation is fed back to control the power, but in the power control method it is impossible to do so.